What should the compression be?

[josh18]

Hi all. I have had a feeling for a while that my trump is low on compression. Partly because when warm the bottom end breather vent I have fitted smokes like a little extra exhaust, and partly because I just think it should pull harder. It revs fantastically and smoothly, it just doesn't seem to have that much pull to go with it.
I ran a compression check today, expecting to see much lower results than from when I last checked it about a year ago. Surprisingly the results are pretty much the same as they were last time.
I did the check dry and again with three squirts of oil in each pot, both times with the throttle wide open.
My results were, from 1 to 6
dry: 150, 140, 140, 140, 140, 135 psi
wet; 165, 165, 160, 160, 160, 150 psi
These results were taken when the gauge stopped reading any higher. I have read that the reading should be taken after just a few revolutions- why is this? My results would have been much lower if I took the readings that way.

Anyway, I am wondering what sort of compression readings I should expect, and does the higher wet readings suggest shot rings, or is that amount acceptable. The car is running a TR5 cam, apart from that the bottom end is pretty much stock. The engine number has a HE on the end, which I am lead to believe means high compression.
Im trying to make a decision on whether or not to rebuild the engine, thinking it needed at least new rings, but now I don't know if these readings are acceptable, or maybe even wrong if I have been taking them wrong.
Cheers guys
Josh

[piman]

Hello Josh,

you have a 15psi spread between best and worse so getting a little too much.

However, if you find the car runs well enough for your needs I would leave well alone. It is really up to you as to what sort of performance you want?

Alec

[josh18]

Hey Alec, yeah I was thinking pretty much the same. It still does really well, doesn't blow smoke or use much oil. I think I'll leave well enough alone for the time being.
Cheers

[GT6 M]

them figs a v low, should ideally be 180+ dry,
most likely is rings,or pistons worn, causing alottt of blow by,
this by yer own admission of alott of crank case pressure .

you ask why it should only be done with a few spins,
well put it this way, you got 150PSI after say 8 cranks, , yer just leading yer sell into a false belief
that yer engine has got higher comp than it actually has.

it would be nice if yer engine could spin owa 8 times to build up compression when its running,  Befoer firing the plug., but it cant, it fires on the first stroke,
and I put loot on yer fust stroke reading will be aboot 80-90 PSI,or even less.

it dont really show up much at Granny an Grandad speeds, but shows badly at higher revs.
think of it like yer air compressor no being able to keep up wid the air tool,
it then slows doon, looses power,  as it runs oot of puff,  

try a running test, as good engine should be showing aboot 130 PSI at idle.
a worn one, maybe 100-115 ish,
its moer this way than cranking, as engine is spinning faster, thus tek,n into play the lost gasses going past rings / valves {ex v }
try it, report back


M

[josh18]

Thanks Marcus. When you say running test do you mean engine running on five cylinders with the gauge in the other? Never heard of that before but makes good sense.

[GT6 M]

yes on 5, at idle espeed
it wont harm  the gauge, as it will just go to the reading its actually running at,

M

[thebrookster]

Been said before, and I'll repeat.

Ignore the figures given on a pressure gauge, none of these devices are calibrated, and so much depends on what spec your engine has. Use these as a guide, no more.

The most important data from this has been pointed out by Alec, it is the deviation between the cylinders. 15 psi is ok for a used engine. You also have a comparison between wet and dry, showing a 15 to 25 psi deviation, again I would consider this relatively normal for a used engine though it does indicate piston rings are starting to wear.

Now taking readings. I personally would go with your approach in taking highest reading,  on a fresh engine this should occur pretty quickly, it it takes longer to build the pressure this again suggests that things are wearing out.

None of your findings immediately cry out engine rebuild, and chances are your engine likely has a few more years in her. However, if you want the best out of it, then a rebuild and recondition is advised. You have symptoms of wear, but not terminal in my opinion.

Cheers,

Phil

[thebrookster]

Oh, and my apologies for the abruptness of my post, I certainly am not trying to be rude or obnoxious! Tis the end of a long and hard day at work where nothing wanted to work correctly and a chief engineer with severe small man syndrome!!!!

Cheers,

Phil

[GT6 M]

A running compression test or dynamic compression test is an important, but little-used and often overlooked engine diagnostic tool. Why is this test so significant? It can be used to pinpoint the cause of a misfire when all the usual tests return normal results. It’s also used by performance tuners to verify that each cylinder is contributing equally to the engine’s overall power output.

When you perform a static compression or cylinder leak-down test, you’re checking the sealing capability of each cylinder. A vacuum test is a great way to determine an engine’s overall breathing capabilities but does not provide results for individual cylinders. The advantage of a running compression test is that it gives you an idea of the volumetric efficiency of each cylinder. In other words, how efficiently each cylinder is pulling air in, retaining it for the correct amount of time, then releasing it into the exhaust. If a cylinder cannot perform these functions properly, the result can be a loss of volumetric efficiency or a density misfire.
How to Perform the Test

    Create a chart similar to this example to record your test readings.
    CYL      STATIC      IDLE      SNAP
    1                    
    2                    
    3                    
    4                    

    Begin with a normal (“static”) compression test on a warm engine to eliminate obvious problems such as badly worn rings, burnt or bent valves or holes in pistons. If you have a diagnostic trouble code, you may know which cylinder is misfiring, but it’s a good idea to test the compression of all the cylinders in that bank to get a good comparison. Record your static compression readings.

    Install all of the spark plugs except the one for the suspected cylinder.

    WARNING: GROUND THAT PLUG WIRE TO PREVENT MODULE DAMAGE, OR WHEN EQUIPPED WITH A COIL-ON-PLUG IGNITION, SIMPLY DISCONNECT THE COIL HARNESS PLUG.

    Disconnect the injector for that cylinder on a port fuel injection system.

    Install your compression tester in the suspected cylinder. The test can be done with the Schrader valve removed, but most techs recommend leaving the valve in the gauge and “burping” the gauge every 5-6 “puffs”.

    Start the engine and take an “Idle” reading. Be sure the idle speed is a consistent 1200 rpm. Record the results.

    Now, from 1200 rpm, snap the throttle to 2500 rpm and release quickly. Reading should rise. Record the results.

    NOTE: Don’t use the gas pedal for this snap acceleration. The idea is to manually open then close throttle as fast as possible, forcing the engine to take a “gulp” of air.

Sample Test Readings

Comparing measurements between cylinders is important. Running compression at idle should be 50-75 PSI (about half cranking compression). Snap throttle compression should be about 80% of cranking compression. Let’s analyze the results from the following sample test readings.

EXAMPLE 1
CYL      STATIC      IDLE      SNAP
1      150      75      85
2      175      80      130
3      160      75      120
4      160      80      120

In Example 1, cylinder #1 has a snap test reading that is much lower than the other cylinders. If a snap reading is low (much less than 80% cranking compression), look for air intake problems such as severe carbon deposits on intake valves, worn cam lobe,worn valve guides and springs, rocker or push rod problems, or “shutter valve” miss-positioned in the runners of a variable runner intake system.  

EXAMPLE 2
CYL      STATIC      IDLE      SNAP
1      150      75      130
2      175      80      130
3      160      75      120
4      160      80      175

In Example 2, cylinder #4 has a higher than normal snap test reading. If a Snap measurement is significantly higher (over 80% of cranking compression), it means the air is not leaving the cylinder efficiently. Look for problems on the exhaust side of that cylinder such as worn cam lobe, bent push rod or collapsed lifter. If the snap readings are all high, look for exhaust restrictions such as a clogged catalytic converter or muffler.
EXAMPLE 3
CYL      STATIC      IDLE      SNAP
1      150      75      130
2      175      40      90
3      160      75      120
4      160      80      175

In Example 3, cylinder #2 has a low Idle and snap test reading. These types of numbers indicate that the cylinder is not holding compression efficiently. Look for issues such as slightly bent or burned valves, excessive carbon build-up on valves or seats, worn valve guides and springs, scored cylinder wall, or a leaking head gasket.

As you can see, information gathered from a running compression test can be very helpful when diagnosing misfires and tuning for total engine performance. It’s a great test to add to your diagnostic arsenal.

M

[josh18]

Thanks Marcus, as always you go the extra mile to help out. I'll give this test a go and see what I come up with.
On a side note, I read up on the answer to my question about why you only crank the engine a few times after I posted yesterday. I found a bloke saying limit the test to 6 puffs and watch how the cylinder pumps up. I repeated my tests with this method and found my results were pretty much the same as my previous ones, and the first compression stroke was doing the bulk of the pumping, suggesting good health.
I'll do the dynamic tests just to make sure but really I think from these tests my engine is not too bad. I think maybe I am just expecting too much from my engine- its only a 2.5 after all, not a v8!
I'll let you guys know what I come up with for the other tests.
Cheers
Josh

[piman]

Hello Marcus,

that's a very interesting test but the more data the more room for confusion and mis-interpretation. Like many things it takes experience to make the most of such tests. That's not to say it isn't worth doing but if a diagnoses is made I would try and double check it with another type of check before diving in and pulling the engine apart?

Alec

[GT6 M]

Alec, ive used this sort of test for ages, but mainly just the  running at idle bit,it were told to  me by a olde mechanic when  first started fiddlating aboot with ingins.
it gives a good indication of what yer engines actually suckin in, /delivering at idle
and can see how the cyl pumps up.

my own engine when good was about 125  on ave
but put a long duration in, and it was doon to 85, as the overlap was so great.
thats why ye need high comp with long cams

M

[piman]

Hello Marcus,

yes the cam is a factor so giving definite figures can be difficult unless you know what cam is fitted. In many ways a leak down test is better as you can often hear where the leak is, just a lot more fiddly to do than a compression test.

Alec